1. Field of the Invention
The present invention relates generally to composite materials that include a thermosetting resin matrix, which is toughened with a thermoplastic toughening agent. More particularly, the present invention is directed to reducing the formation of solvent-induced micro cracks that are known to occur in such thermoplastic toughened resin matrices.
2. Description of Related Art
The two principal components of a typical composite material are the polymeric resin matrix and the fibrous reinforcement. In the aerospace industry, thermosetting resins are commonly used as one of the major ingredients in a variety of resin matrices. Epoxy resins, bismaleimide resins and cyanate ester resins are common thermosetting resins. It is a popular practice to “toughen” these thermosetting resins by adding varying amounts of a thermoplastic toughening agent. Polyether sulfone (PES), polyether ethersulfone (PEES) and polyether imide (PEI) are a few examples of thermoplastic toughening agents that have been routinely added to thermosetting resins.
Thermosetting resins, like many other polymeric resins, can be vulnerable to attack by certain liquids, such as solvents, that come into contact with the cured resin. For example, many primers and paints in the aerospace industry use a variety of solvents, such as methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), xylene, toluene, isobutyl acetate, ethanol, n-butyl acetate, isopropyl alcohol, glycol ethers and glycol esters. Many of these solvents are known to attack the resin surface during application of the primer and/or paint to the finished composite part. The result of this attack is the formation of micro cracks that can penetrate to varying depths within the resin. These micro cracks can have a substantial and deleterious effect on the physical strength of the finished composite part.
Composite parts may also be exposed to a variety of solvents and caustic liquids that are used to clean the composite part or remove old paint prior to re-painting of the part. Paint stripping liquids typically include strong solvents, such as acetone, MEK and chlorinated hydrocarbons, which are capable of forming micro cracks in the resin matrix. In addition, the resin matrix may unintentionally be exposed to micro crack-forming solvents or liquids during the lifetime of the composite part. For example, the resin matrix may be exposed to solvents or other possibly harmful liquids due to leaks in a given fluid system where the composite part may be located.
An epoxy-based matrix resin that includes PES and/or PEES or their copolymers as the thermoplastic toughening agent is a rather common resin matrix for aerospace applications. In many cases, however, the final toughened epoxy resin is susceptible to solvent attack and the formation of micro cracks with the resultant negative effect on mechanical stability of the composite part. One approach to avoid the undesirable formation of micro cracks is to use chemically reactive grades of PES and/or PEES. For example, reduction in micro crack formation has been achieved by using amino-terminated PES instead of hydroxyl-terminated PES that is usually used to toughen epoxy resins. However, amino-terminated PES is more difficult and expensive to prepare than the less chemically reactive hydroxyl-terminated PES.
In view of the above, there is a continuing need to develop a simple, efficient and cost effective way to eliminate, or at least substantially reduce, the susceptibility of thermoplastic toughened thermosetting matrix resins to solvent-induced micro cracking.